Bridge or chopper circuits are frequently employed in applications such as motor drivers and switched-mode power supplies. In such circuits, high currents are switched repeatedly to an output filter or load by means of power switching devices, such as thyristors or power transistors, which are controlled by the application of suitable drive signals to their control electrodes. The d.c. power supply for such circuits is commonly derived by rectifying line voltage with no intervening transformer.
The drive signals for the power switching devices are, in turn, derived from switching logic signals generated by switching control logic. Such logic signals, even if of suitable form, are not normally applied directly to the power switching devices because the control logic is normally part of circuitry referenced to ground and isolated from the local line voltage. Accordingly, they, or drive signals derived from them, are usually applied via an isolated interface, such as an opto-isolator or a transformer.
One example of the use of an opto-isolator is described in U.S. Pat. No. 4,401,933 which shows a half bridge circuit for driving a single phase induction motor. Switching logic signals are applied via an opto-isolator to amplifiers which provide turn-on base drive signals alternately to the pair of series-connected power transistors forming the half bridge. The switching logic is thus isolated from the power switching devices. The amplifiers, however, are on the power side of the isolating interface, and must be provided with independent floating power supplies connected to the appropriate transistor emitter in order to be able to switch the power transistors correctly, since the d.c. supply to the transistors is floating.
One example of a transformer coupled arrangement for supplying power to a load is shown in U.S. Pat. No. 3,820,008 in which the current through a switching transistor, connected across an unregulated floating supply, is pulse width modulated and filtered to provide a regulated supply to the load. The current flowing in the primary circuit of a pulse transformer is modulated in response to control logic signals. The induced current in the secondary thus provides isolated base drive current to the power switching transistor which is unaffected by changes in the collector and emitter potentials of the switching transistor due to the floating nature of the supply.
In the case of this and other switched-mode power supplies, the frequency of modulation is commonly of the order of tens of kHz. At such frequencies relatively small transformers can be used and relatively fast switching can be achieved although auxiliary transformer windings may also be employed to speed up the switching or improve the modulation waveforms.
In the case of a motor driver, however, the switching frequencies required are considerably lower and are typically of the order of fifty to a few hundred Hz. Transformers with sufficiently large magnetising inductance to accommodate the lowest drive frequencies (50 Hz) and with sufficiently small leakage inductance to provide fast switching edges do not exist, yet fast switching is essential to avoid excessive transient power dissipation in the power switching transistors as they turn on and off.